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Mechanical Engineer´s Handbook P22

Fig. 18.12 Surface flaw shape parameter. (From Ref. 22. Adapted by permission of Prentice-Hall, Inc., Englewood Cliffs, New Jersey.) To approximate the effects of strain hardening, a flow stress cr0, taken to be an average of the yield and ultimate strengths, is often used when computing the plastic collapse stress. The plastic collapse stress ac is that applied stress which produces cr0 across the remaining uncracked ligament, and is the maximum applied stress that a perfectly plastic material can sustain. This stress may be determined using a limit load analysis. In general, the plastic collapse stress is a function of geometry,. | 396 FAILURE CONSIDERATIONS Fig. 18.12 Surface flaw shape parameter. From Ref. 22. Adapted by permission of Prentice-Hall Inc. Englewood Cliffs New Jersey. To approximate the effects of strain hardening a flow stress ao taken to be an average of the yield and ultimate strengths is often used when computing the plastic collapse stress. The plastic collapse stress ac is that applied stress which produces ro across the remaining uncracked ligament and is the maximum applied stress that a perfectly plastic material can sustain. This stress may be determined using a limit load analysis. In general the plastic collapse stress is a function of geometry type of loading type of support boundary conditions and through-thickness constraint plane stress or plane strain .6 25 For a single through-thickness crack of length a in a strip with width b loaded in tension see Fig. 18.9 if end rotations are restrained the plastic collapse stress under plane stress conditions may be approximated by25 ac o l - alb 18.39 18.5 FATIGUE AND STRESS CONCENTRATION Static or quasistatic loading is rarely observed in modem engineering practice making it essential for the designer to address himself or herself to the implications of repeated loads fluctuating loads and rapidly applied loads. By far the majority of engineering design projects involve machine parts Fig. 18.13 Failure assessment diagram. 18.5 FATIGUE AND STRESS CONCENTRATION 397 subjected to fluctuating or cyclic loads. Such loading induces fluctuating or cyclic stresses that often result in failure by fatigue. Fatigue failure investigations over the years have led to the observation that the fatigue process actually embraces two domains of cyclic stressing or straining that are significantly different in character and in each of which failure is probably produced by different physical mechanisms. One domain of cyclic loading is that for which significant plastic strain occurs during each cycle. This domain is associated with high .